The agency chief was keen to head off criticism that the proposals amounted to a re-tread of those missions: "Much of it looks the same, but that's because the physics of atmospheric entry haven't changed recently," he said.

"We really proved once again how much of it all the Apollo guys got right."

Missions to the Moon will use a lander craft

Nasa is charged with implementing the vision for space exploration, laid out in January 2004 by President George W Bush.

This vision aims to return humans to the Moon, and then to use it as a staging point for a manned mission to Mars.

"We believe this architecture... achieves those goals in the most cost-effective, efficient manner that we could do it," said Dr Griffin in a news briefing at Nasa headquarters in Washington DC.

The space shuttle is to be retired by 2010 in order to pay for its replacement, the Crew Exploration Vehicle (CEV) - to be ready by 2012. This vehicle would be shaped like the Apollo command and service modules, but three times larger, and able to take four astronauts to the Moon at a time.

Fly me to the Moon

Nasa would begin the first lunar expedition by launching a lunar landing capsule and a propulsion stage atop a new heavy-lift rocket.

Mike Griffin said the missions would use existing money

This will consist of a lengthened shuttle external tank and a pair of solid rocket boosters capable of putting up to 125 metric tonnes in orbit - about one and a half times the weight of a shuttle orbiter.

The cargo it carries could wait for up to 30 days in orbit for the astronauts to launch aboard their CEV.

Carrying a crew of four, the CEV would blast off atop a single solid-rocket booster consisting of four segments - exactly like those flown with the shuttle.

Once in orbit, the manned orbiter would dock with the lunar lander and the propulsion stage and begin the journey to the Moon.

HAVE YOUR SAY

It will be wonderful to be able to look up and know that we have dared to explore the unknown once more

After a three-day journey, the four astronauts would climb into the lander craft, leaving the CEV to wait for them in lunar orbit.

After landing and exploring the surface for seven days, the crew would then blast off in a portion of the lander, dock with the capsule and return to Earth, parachuting through the atmosphere to dry land.

Nasa says it will be able to recover the entry capsule, replace the heat shield and re-launch the craft up to 10 times.

Reconstruction costs

Dr Griffin dismissed suggestions that reconstruction of the Gulf Coast in the wake of Hurricane Katrina could derail the programme.

The capsule could be re-used up to 10 times

"We must deal with our short-term problems while not sacrificing our long-term investments in our future," the Nasa chief said.

"When we have a hurricane, we don't cancel the Air Force. We don't cancel the Navy. And we're not going to cancel Nasa."

But Representative Bart Gordon, a Tennessee Democrat on the US House Science Committee, said in a statement: "This plan is coming out at a time when the nation is facing significant budgetary challenges.

"Getting agreement to move forward on it is going to be heavy lifting in the current environment, and it's clear that strong presidential leadership will be needed."

Nasa also envisions the possibility of building a semi-permanent lunar base, where astronauts would make use of the Moon's natural resources for water and fuel.

(1) A heavy-lift rocket blasts off from Earth carrying a lunar lander and a "departure stage"

(2) Several days later, astronauts launch on a separate rocket system with their Crew Exploration Vehicle (CEV)

(3) The CEV docks with the lander and departure stage in Earth orbit and then heads to the Moon

(4) Having done its job of boosting the CEV and lunar lander on their way, the departure stage is jettisoned

(5) At the Moon, the astronauts leave their CEV and enter the lander for the trip to the lunar surface

(6) After exploring the lunar landscape for seven days, the crew blasts off in a portion of the lander

(7) In Moon orbit, they re-join the waiting robot-minded CEV and begin the journey back to Earth

(8) On the way, the service component of the CEV is jettisoned. This leaves just the crew capsule to enter the atmosphere

(9) A heatshield protects the capsule; parachutes bring it down on dry land, probably in California